This invention relates to an improved flextensional sonar transducer assembly constructed to maximize operating depth by compensating for creep or movement, during use, between the outer flexural shell and the inner transducer driver.
Flextensional sonar transducers have been in use for some time in such applications as underwater sonar signal transmission detection. See U.S. Pat. Nos. 3,274,537, 3,277,433 and 4,462,093. Flextensional transducers typically employ a stack of piezoelectric transducer elements interspersed with electrically conducting plates for stressing the elements and for picking up electrical current produced by the elements, and an outer elliptically-shaped shell wrapped about the stack. The stack of piezoelectric elements generally extend along the major axis of the ellipse defined by the outer shell. When an alternating voltage is applied to the conducting plates, the stack of piezoelectric elements are caused to be displaced in the direction of the major axis in proportion to the instantaneous value of the voltage. The vibration and displacement of the stack is transmitted to the shell which amplifies the vibration along the minor access of the ellipse to produce the sonar signals. That is, as the stack expands to expand the major axis of the ellipse, the long walls of the ellipse perpendicular to the minor axis of the ellipse contract, and as the stack contracts to expand the long walls of the ellipse, vibration of the shell necessary to generate the sonar signals is produced. In an alternative arrangement of a flextensional transducer, a magnetostrictive drive element may replace the piezoelectric stack.
The elliptical shells used in flextensional transducers are typically pre-formed of filament wound composites or metals such as glass reinforced plastic or aluminum. In order to incorporate the stack of piezoelectric elements in the shell, the shell is compressed along its minor access by means of a press, and then the piezoelectric stack is inserted in the shell to coincide with the major access. The ends of the stack are attached to corresponding apices of the shell so that on removal of the compressive force from along the minor access, a residual tension remains in the shell to retain the stack and apply a predetermined compressive stress to the stack. Construction of the assembly in this fashion requires that the piezoelectric stack and elliptical shell be prepared to close tolerances both to allow for easy insertion of the stack within the compressed shell, and to retain tight contact between the stack and the shell upon removal of the compressive force.
An additional feature of some prior art flextensional transducers is the inclusion of a so-called pre-stress compression band, made for example of filament wound material, wrapped about the piezoelectric stack and circumscribed by the elliptical shell. This compression band allows for the application of a precise pre-stress (compression) to the piezoelectric stack. Such application of a pre-stress to the stack allows for accurate operation of the transducer in deep water. When the transducer assembly is deployed into water, the increasing hydrostatic pressure with depth reduces the prestress on the stack (since the elliptical shell tends to be compressed along the minor axis thus removing shell pressure along the major axis) and eventually a depth may be reached beyond which the transducer cannot be driven without damage. Use of the compression band enables reproduction of the same pre-stress levels from one transducer assembly to another.
One problem with the prior art flextensional transducers, whether the transducers employ the stress band or not, is that they can be rendered inoperable if used repeatably at significant water depths. This occurs when the hydrostatic pressure reduces the minor axis of the elliptical shell and extends the major axis of the shell to exceed the pre-stress of the shell so that the shell creeps or moves (elongates) relative to and then becomes detached and decoupled from either the compression band (if one is used) or from the ends of the piezoelectric stack (if a compression band is not used). Then, when the transducer assembly is brought from the water depth, the shell does not return to its original position of coupling between the stack and the shell so the assembly would not be reusable, at least at shallow depths. At deeper depths, the shell might be compressed enough along the minor axis to "squeeze" onto and couple with the band or stack and be operable. But each such use at the deeper depths causes further creep and decoupling until eventually the assembly becomes inoperable at any depth (at least any depth of interest).